# Dica 44 - Django: F() expression [![](/files/-MfuAOwUjEsk4qZiRsOx)](https://youtu.be/YDCeA3rNcno) O [F() expression](https://docs.djangoproject.com/en/3.2/ref/models/expressions/#f-expressions) é uma expressão que retorna a representação do valor do campo, ou seja, é o valor do campo propriamente dito. Vamos criar 3 novas apps: ``` cd myproject python ../manage.py event python ../manage.py product python ../manage.py ecommerce ``` Edite `event/apps.py` ```python from django.apps import AppConfig class EventConfig(AppConfig): default_auto_field = 'django.db.models.BigAutoField' name = 'myproject.event' ``` Edite `event/models.py` ```python from django.db import models class Room(models.Model): name = models.CharField('nome', max_length=100, unique=True) num_participants = models.PositiveSmallIntegerField('quantidade de participantes') # noqa E501 num_chairs = models.PositiveSmallIntegerField('quantidade de cadeiras') class Meta: ordering = ('name',) verbose_name = 'sala' verbose_name_plural = 'salas' def __str__(self): return self.name ``` Edite `event/admin.py` ```python from django.contrib import admin from .models import Room @admin.register(Room) class RoomAdmin(admin.ModelAdmin): list_display = ('name', 'num_participants', 'num_chairs') search_fields = ('name',) ``` Rode ``` python manage.py shell_plus ``` ```python # Room.objects.all().delete() rooms = [ ('Evento 1', 500, 400), ('Evento 2', 500, 550), ('Evento 3', 600, 500), ('Evento 4', 680, 700), ('Evento 5', 1000, 990), ('Evento 6', 1200, 500), ('Evento 7', 1000, 450), ] for room in rooms: Room.objects.create(name=room[0], num_participants=room[1], num_chairs=room[2]) # Filtra as salas cuja quantidade de participantes seja maior que a quantidade de cadeiras. rooms = Room.objects.filter(num_participants__gt=F('num_chairs')) for room in rooms: print(room.num_participants, room.num_chairs, room.num_participants - room.num_chairs) # Calcula a diferença entre participantes e cadeiras. rooms = Room.objects.annotate(difference=F('num_participants') - F('num_chairs')).filter(num_participants__gt=F('num_chairs')) for room in rooms: print(room.num_participants, room.num_chairs, room.difference) # Filtra as salas cuja quantidade de participantes seja maior que o dobro da quantidade de cadeiras. rooms = Room.objects.filter(num_participants__gt=F('num_chairs') * 2) for room in rooms: print(room.num_participants, room.num_chairs, room.num_chairs * 2) ``` Edite `product/apps.py` ```python from django.apps import AppConfig class ProductConfig(AppConfig): default_auto_field = 'django.db.models.BigAutoField' name = 'myproject.product' ``` Edite `product/models.py` ```python from django.db import models class Product(models.Model): title = models.CharField('título', max_length=100, unique=True) price = models.DecimalField('preço', max_digits=7, decimal_places=2) manufacturing_date = models.DateField('data de fabricação', null=True, blank=True) # noqa E501 due_date = models.DateField('data de vencimento', null=True, blank=True) class Meta: ordering = ('title',) verbose_name = 'produto' verbose_name_plural = 'produtos' def __str__(self): return self.title ``` Edite `product/admin.py` ```python from django.contrib import admin from .models import Product @admin.register(Product) class ProductAdmin(admin.ModelAdmin): list_display = ('title', 'price', 'manufacturing_date', 'due_date') search_fields = ('title',) ``` Rode ``` python manage.py shell_plus ``` ```python products = [ { 'title': 'queijo fresco', 'price': '8.99', 'manufacturing_date': '2021-08-01', 'due_date': '2021-08-03', }, { 'title': 'sorvete de manga', 'price': '12', 'manufacturing_date': '2021-08-10', 'due_date': '2021-10-10', }, { 'title': 'leite integral', 'price': '5.12', 'manufacturing_date': '2021-08-02', 'due_date': '2021-08-06', }, { 'title': 'pão de forma', 'price': '7.45', 'manufacturing_date': '2021-08-01', 'due_date': '2021-08-06', }, ] Product.objects.all().delete() aux_list = [] for item in products: obj = Product( title=item['title'], price=item['price'], manufacturing_date=item['manufacturing_date'], due_date=item['due_date'], ) aux_list.append(obj) Product.objects.bulk_create(aux_list) from datetime import timedelta # Retorna os produtos cuja data de validade seja inferior a 5 dias. Product.objects.annotate(expiration_days=F('due_date') - F('manufacturing_date')).filter(expiration_days__lt=timedelta(days=5)) products = Product.objects.all() for product in products: print(product.title, (product.due_date - product.manufacturing_date).days) ``` Edite `ecommerce/apps.py` ```python from django.apps import AppConfig class EcommerceConfig(AppConfig): default_auto_field = 'django.db.models.BigAutoField' name = 'myproject.ecommerce' ``` Edite `ecommerce/models.py` ```python from django.db import models from myproject.core.models import TimeStampedModel from myproject.product.models import Product class Order(TimeStampedModel): nf = models.CharField('nota fiscal', max_length=100, unique=True) class Meta: ordering = ('nf',) verbose_name = 'ordem de compra' verbose_name_plural = 'ordens de compra' def __str__(self): return self.nf class OrderItems(models.Model): order = models.ForeignKey( Order, on_delete=models.SET_NULL, verbose_name='ordem', related_name='order_items', null=True, blank=True ) product = models.ForeignKey( Product, on_delete=models.SET_NULL, verbose_name='produto', related_name='product_items', null=True, blank=True ) quantity = models.PositiveIntegerField('quantidade') price = models.DecimalField('preço', max_digits=7, decimal_places=2) class Meta: ordering = ('pk',) def __str__(self): return f'{self.pk} - {self.order.pk} - {self.product}' ``` Edite `ecommerce/admin.py` ```python from django.contrib import admin from .models import Order, OrderItems admin.site.register(Order) @admin.register(OrderItems) class OrderItemsAdmin(admin.ModelAdmin): list_display = ('__str__', 'quantity', 'price') ``` Rode ``` python manage.py seed product ecommerce --number=10 ``` Depois abra o shell\_plus ``` python manage.py shell_plus ``` ```python # Retorna o subtotal do preço vezes a quantidade de cada produto. items = OrderItems.objects.annotate(subtotal=F('product__price') * F('quantity')) for item in items: print(f'{item.quantity}\t{item.product.price}\t{round(item.subtotal, 2)}') ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://www.dicas-de-django.com.br/044-django-f-expression.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.